SNSPD with integrated aluminum nitride seed or waveguide layer

ABSTRACT

A superconducting nanowire single photon detector (SNSPD) device includes a substrate having a top surface, an optical waveguide on the top surface of the substrate to receive light propagating substantially parallel to the top surface of the substrate, a seed layer of metal nitride on the optical waveguide, and a superconductive wire on the seed layer. The superconductive wire is a metal nitride different from the metal nitride of the seed layer and is optically coupled to the optical waveguide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Application Ser. No.62/969,637, filed on Feb. 3, 2020, the disclosure of which isincorporated by reference.

BACKGROUND Technical Field

The disclosure concerns a superconducting nanowire single photondetector (SNSPD) that includes a seed layer below the metal nitride thatprovides the superconductive material.

Background Discussion

In the context of superconductivity, the critical temperature (T_(C))refers to the temperature below which a material becomessuperconductive. Niobium nitride (NbN) is a material that can be usedfor superconducting applications, e.g., superconducting nanowire singlephoton detectors (SNSPD) for use in quantum information processing,defect analysis in CMOS, LIDAR, etc. The critical temperature of niobiumnitride depends on the crystalline structure and atomic ratio of thematerial. For example, referring to FIG. 1 , cubic δ-phase NbN has someadvantages due to its relatively “high” critical temperature, e.g.,9.7-16.5° K.

Niobium nitride can be deposited on a workpiece by physical vapordeposition (PVD). For example, a sputtering operation can be performedusing a niobium target in the presence of nitrogen gas. The sputteringcan be performed by inducing a plasma in the reactor chamber thatcontains the target and the workpiece.

SUMMARY

In one aspect, a superconducting nanowire single photon detector (SNSPD)device includes a substrate having a top surface, an optical waveguideon the top surface of the substrate to receive light propagatingsubstantially parallel to the top surface of the substrate, a seed layerof metal nitride on the optical waveguide, and a superconductive wire onthe seed layer. The superconductive wire is a metal nitride differentfrom the metal nitride of the seed layer and is optically coupled to theoptical waveguide.

In another aspect, a superconducting nanowire single photon detector(SNSPD) device includes a substrate having a top surface, a metalnitride optical waveguide on the top surface of the substrate to receivelight propagating substantially parallel to the top surface of thesubstrate, and a superconductive wire on the optical waveguide. Thesuperconductive wire is a metal nitride selected from the groupconsisting of niobium nitride, titanium nitride, and niobium titaniumnitride. The metal nitride of the optical waveguide is different fromthe metal nitride of the superconductive wire.

Implementations may provide, but are not limited to, one or more of thefollowing advantages. A device based on absorption of photons by asuperconductive material, e.g., an SNSPD, can have high photonabsorption efficiency while also achieving high material quality for thesuperconductive layer, e.g. the niobium nitride, and thus a highercritical temperature. This permits fabrication of devices, e.g., SNSPD,with superconductive wires with a higher critical temperature. Thelarger difference between the operating temperature (2-3° K) and thecritical temperature provides superior detection efficiency, lower darkcount, and possibly faster temporal response.

It should be noted that “superconductive” indicates that the materialbecomes superconducting at the operating temperature of the device,e.g., 2-3° K. The material is not actually superconducting duringfabrication of the device at or above room temperature or when thedevice is not being cooled for operation.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other potentialaspects, features, and advantages will become apparent from thedescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagram illustrating phase of niobium nitride as a function ofprocessing temperature and atomic percentage nitrogen.

FIG. 2A is a schematic top view of a SNSPD that includes a distributedBragg reflector.

FIG. 2B is a schematic cross-sectional side view of the device of FIG.2A.

FIG. 3 is a schematic illustration of the operation of a SNSPD.

FIG. 4 is a schematic cross-sectional side view of a SNSPD that includesa distributed Bragg reflector and an aluminum nitride seed layer.

FIG. 5 is a graph of reflectance as a function of wavelength for twoSNSPD designs.

FIG. 6 is a graph of critical temperature as a function of thickness ofa NbN layer, with and without an aluminum nitride seed layer.

FIG. 7 is a flow chart of a method of fabricating a SNSPD.

FIG. 8A is a schematic top view of a SNSPD that includes a waveguide.

FIG. 8B is a schematic cross-sectional side view of the device of FIG.8A.

FIG. 9 is a schematic cross-sectional side view of a SNSPD that includesa waveguide and an aluminum nitride seed layer.

FIG. 10 is a schematic cross-sectional side view of a SNSPD thatincludes a waveguide formed of aluminum nitride.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIGS. 2A and 2B illustrate top and side views, respectively, of aconventional superconducting nanowire single photon detector (SNSPD)device 10. The SNSPD device 10 can include at least one superconductivewire 12 disposed on a support structure 20. The superconductive wire 12can be connected between conductive electrodes 14. The superconductivewire 12 can be arranged in a meandering pattern, e.g., a back-and-forthparallel lines, on the supporting structure 20. In some implementations,multiple wires 12 are connected in parallel between the electrodes 14,with each wire 12 covering a separate area 16, but there could be just asingle wire 12 covering the entire detection area of the device 10. Inaddition, many other patterns are possible, e.g., zigzag or doublespiral. The superconductive wire can be considered a nanowire, e.g., canhave a width of about 30 nm and a thickness of about 10 nm.

The support structure 20 can include a substrate 22, e.g., a siliconsubstrate, and a mirror structure 24 disposed on the substrate 22. As anexample, the mirror structure 24 can be a distributed Bragg reflector(DBR) that includes multiple pairs of layers formed of high refractiveindex and low refractive index materials.

A conventional SNSPD is operated with a photon (illustrated by lightbeam 30) approaching from the top of the device 10, e.g., with normalincidence relative to the substrate 20. A simple device would be havethe NbN nanowires disposed directly on the silicon substrate (withoutthe mirror structure). Because the NbN nanowires are typically very thinin a SNSPD device, most of the light is not absorbed by the NbNnanowire. To boost the light absorption efficiency, the mirror structure24, e.g., the distributed Bragg reflector, is incorporated in the device10 between the substrate 20 and the wires 12. In this case, incidentphotons that are not initially absorbed can be absorbed on reflection,so the photons have a higher probability to be captured by the NbNnanowires.

Referring to FIG. 3 , the working principle of the SNSPD device is thatthe to-be-detected photon comes from top and shines on the SNPSD.Absorption of the photon creates a hot spot on the NbN nanowire whichraises the temperature of the NbN above critical temperature so that aportion of the wire is no longer in the superconductive state. A regionaround the hot spot can experience current crowding, resulting in ahigher current density than the critical current density, which candisrupt the superconductive state for the entire wire. The change in theNbN wire from the superconducting state to the normal resistive statecan be electrically detected by flowing a current through the device andmonitoring voltage differences between the electrodes.

NbN based SNSPDs are mainly used for time-correlated single-photoncounting (TCPSC) related applications in the visible and infraredwavelength. For example, SNSPDs are used in quantum metrology (quantumkey generation, quantum emitter) and optical quantum computing(detection module) due to their high efficiency, low dark count, lowtiming jitter, and fast recover time. They can be also used as detectorsin classical space-to-ground communications and time-of-flight LIDARsystem.

In the visible wavelength range, Si avalanche photodiodes (APDs) aretypically used. The system detection efficiency is not ideal, e.g., isabout 70%, and these devices are hard to integrate with chip-scaledevices.

In the infrared wavelength range, InGaAs APDs are a candidate for manyapplications. But these devices usually suffer from high dark countrates and even lower system detection efficiency (<30%) with limiteddetection speed. Compared to APDs, SNSPDs, have superior performancewhich includes low timing jitter (<20 ps), fast recover time, highdetection efficiency (>85%), and low dark count rates (˜a few Hz).

As noted above, niobium nitride, particularly δ-phase NbN, has someadvantages as a superconductive material. However, δ-phase NbN can bedifficult to deposit at a satisfactory quality. A seed layer, e.g., analuminum nitride (AlN) layer, below the (super)conductive layer can helpimprove the critical temperature of the NbN layer. The aluminum nitride(AlN) seed layer can also improve the critical temperature of TiN andNbTiN layers, and may also be helpful for other metal nitride layers. Inparticular, the aluminum nitride layer can be integrated into a SNSPDdevice, and in particular be integrated into the mirror structure orwaveguide of a SNSPD device. This permits a higher crystal quality metalnitride detector (thus higher critical temperature, and thus betterdevice performance) to be achieved while also enabling high lightabsorption efficiency.

FIG. 4 illustrates a cross-sectional side view of a superconductingnanowire single photon detector (SNSPD) device 100. The SNSPD device 100can be similar to the device described above with respect to FIGS. 2Aand 2B, except as described below.

The SNSPD includes a substrate 22, which can be a dielectric material,e.g., sapphire, SiO₂, fused silica, or quartz, or a semiconductormaterial, e.g., silicon, gallium nitride (GaN) or gallium arsenide(GaAs).

A distributed Bragg reflector (DBR) 24 is fabricated on top of thesubstrate 22. The DBR 24 includes multiple bi-layers 26, e.g., two toeight bi-layers, e.g., seven bi-layers. Each bi-layer 26 includes alower layer 26 a of a first material having a first index of refraction(the “low index of refraction”) and an upper layer 26 b of a secondmaterial having a second index of refraction (the “high index ofrefraction”) that is greater than the first index refraction. Thethicknesses and materials (and thus indices of refraction) in thebilayers 26 are selected to increase reflection in a selected wavelengthor wavelength band. For example, the DBR may be optimized for reflectionof light of about 1500-1600 nm, as 1550 nm is a widely used wavelengthin optical communication systems.

The first material and the second material can both be selected fromTable 1, subject to the restriction that the second material has ahigher index of refraction than the first material.

TABLE 1 Refractive index Material (for 1550 nm light) a-Si ~3.4-3.5 TiO₂~2.2-2.3 Nb₂O₅ ~2.1-2.2 Ta₂O₅ ~2.05-2.15 AlN ~1.95-2.05 Si₃N₄ ~1.9-2.0SiO₂ ~1.4-1.5

Covering the upper layer 26 b of the topmost bilayer 26 of thedistributed Bragg reflector, e.g., in direct contact with the upperlayer 26 b, is a metal nitride seed layer 102. The seed layer 102 andthe superconductive wires 12 are nitrides of different metals. Inparticular, the seed layer 102 can be aluminum nitride (AlN), as thisimproves the critical temperature of NbN. However, hafnium nitride(HfN), gallium nitride (GaN) might also be suitable. The metal nitrideseed layer 102 can have a thickness of about 4 to 50 nm, e.g., about 5nm or about 10 nm or about 20 nm thickness. The seed layer 102 can havea (002) c-axis crystal orientation. The seed layer 102 is notsuperconducting at the operating temperature of the device 100. The seedlayer 102 can be deposited by a standard chemical vapor deposition orphysical vapor deposition process.

In some implementations, the high index of refraction material of theupper layer 26 b is Ta₂O₅, e.g., of about 182 nm thickness, and the lowindex of refraction material of the lower layer 26 a is SiO₂, e.g., ofabout 263 nm thickness. The reflectance, as simulated by opticalmodelling software, of a stack of seven such bilayers with an AlN seedlayer of 20 nm thickness is shown by curve 120 in FIG. 5 .

In an embodiment of particular interest, the seed layer 102 is aluminumnitride, and the low-index material, i.e., the material of each lowerlayer 26 a, is also aluminum nitride. This permits the seed layer 102 tobe fabricated using the same processing conditions as the lower-layersin the distributed Bragg reflector 24, and thus simplifies processingrequirements. In some implementations, the high index of refractionmaterial is amorphous silicon (a-Si), e.g., of about 111 nm thickness,and the low index of refraction material is AlN, e.g., of about 197 nmthickness. The reflectance, as simulated by optical modelling software,of a stack of seven such bilayers with an AlN seed layer of 20 nmthickness is shown by curve 122 in FIG. 5 .

The superconductive wires 12 are formed on, e.g., in direct contactwith, the seed layer 102. The wires are formed of niobium nitride (NbN),titanium nitride (TiN), or niobium titanium nitride (Nb_(X)Ti_(1-X)N).The wires 12 can have a width of about 25 to 250 nm, e.g., about 60 nm,and a thickness of 4 to 50 nm, e.g., about 5 nm or about 10 nm or about20 nm.

The seed layer 102 helps improve the critical temperature of thealuminum nitride, especially when the aluminum nitride layer is thin.For example, FIG. 6 illustrates the measured critical temperature T_(C)(in Kelvin) as a function of thickness of a NbN layer. Curve 130illustrates the critical temperature without an aluminum nitride seedlayer, and curve 132 illustrates the critical temperature with analuminum nitride seed layer (for a simplified stack of a silicon wafer,AlN seed layer, and NbN layer). Alternatively or in addition, the seedlayer 102 can improve adhesion between the aluminum nitride layer 102and the upper layer 26 b of the distributed Bragg reflector 24.

FIG. 7 is a flowchart of a method 200 of fabrication of the device 100of FIG. 4

Initially, the distributed Bragg reflector (DBR) 24 is deposited on asubstrate 100 (step 202). The substrate can be, for example, a siliconwafer. Although illustrated as a single block, the substrate 22 couldinclude multiple underlying layers. The DBR 24 can be deposited byalternating deposition of the high and low index materials using astandard chemical vapor deposition or physical vapor deposition process.

Next, the seed layer 102 is deposited on the DBR 24 (step 204). Asmentioned above, the seed layer 102 can be aluminum nitride. The seedlayer can be deposited using a standard chemical vapor deposition orphysical vapor deposition process. Exemplary processing parameters are apower applied to the sputtering target of 1-5 kW, a total pressure(nitrogen and inert gas) of 2 to 20 mTorr with nitrogen gas and inertgas supplied at a ratio between 3:100 and 1:6, a wafer temperature of200-500° C., and no bias voltage applied to the wafer. In someimplementations, the seed layer 102 is deposited in the same processingchamber that is used to deposit the DBR 24, e.g., by switching in a newtarget. This permits higher throughput manufacturing. Alternatively, thesubstrate can be transported to a different deposition chamber withoutbreaking vacuum. Either case permits the seed layer to be depositedwithout exposure of the DBR to atmosphere and with lower risk ofcontamination.

Next, the metal nitride layer 12, e.g., the niobium nitride (NbN),titanium nitride (TiN), or niobium titanium nitride (Nb_(X)Ti_(1-X)N),is deposited on the seed layer (step 206). The metal nitride layer 12can be deposited using a standard chemical vapor deposition or physicalvapor deposition process. Exemplary processing parameters are a basepressure of 1e-8 Torr, a power applied to the target of 1-3 kW, a totalpressure during processing of 5-7 mTorr, a wafer temperature of 400 C,no bias voltage applied to the wafer, and a percentage of the gas as N₂sufficient to achieve cubic δ-phase NbN. In some implementations, themetal nitride layer 12 is deposited in the same processing chamber thatis used to deposit the seed layer 102, e.g., by switching in a newtarget. This permits higher throughput manufacturing. Alternatively, thesubstrate can be transported to a different deposition chamber withoutbreaking vacuum. This permits the metal nitride layer to be depositedwithout exposure of the seed layer to atmosphere and with lower risk ofcontamination.

After the metal nitride layer 12 is deposited, a capping layer 104 canbe deposited on the metal nitride layer 12 (step 208). The capping layer104 serves as a protective layer, e.g., to prevent oxidation of themetal nitride layer 12 or other types of contamination or damage. Thecapping layer 104 can be dielectric or conductive but is notsuperconductive at the operating temperature of the device 100. Thecapping layer 104 can be amorphous silicon (a-Si). In someimplementations, the capping layer 104 is a nitride of a differentmaterial from the metal of the metal nitride used for thesuperconductive layer 12. Examples of materials for the capping layer104 include AlN, Al₂O₃, SiO₂, and SiN. The capping layer 104 can bedeposited by a standard chemical vapor deposition or physical vapordeposition process.

Etching can be used to form trenches 108 through at least the metalnitride layer 12 to form the conductive wires 12 or other structuresneeded for the device 100 (step 210). Although FIG. 4 illustrates thetrench as extending through the metal nitride layer 12 and capping layer104, other configurations are possible. As an example, the trenches canextend partially into or entirely through the seed layer 102. However,the trenches should not extend into the mirror structure 24.

Another form of superconducting nanowire single photon detector (SNSPD)device includes a waveguide to input photons into the detector along anaxis generally parallel to the surface of the substrate. FIGS. 8A and 8Billustrate a conventional SNSPD 50 having such a waveguide. The SNSPD 50can include ate least one superconductive wire 52 disposed on a supportstructure 60. The support structure can include a substrate 62, e.g., asilicon substrate, a dielectric layer 64 on the substrate 62, and awaveguide 66 disposed on the dielectric layer 64. The dielectric layer64 is a first material having a first refractive index, and thewaveguide 66 is a second material having a second refractive index thatis higher than the first refractive index.

The superconductive wire 52 can be considered a nanowire, e.g., can havea width of about 30 nm and a thickness of about 10 nm. Thesuperconductive wire(s) 52 can be arranged to form a plurality ofparallel lines, with adjacent lines connected at alternating ends.Although FIG. 8A illustrates four parallel lines, the device could havejust two parallel lines, e.g., a U-shaped wire, or a greater number oflines.

Photons, shown by light beam 70, are injected into the device from theside, e.g., generally parallel to the top surface of the substrate 62,through the waveguide layer 66. In particular, the photons can enteralong an axis (shown by arrow A) generally parallel to the parallellines of the wire 52. In addition, along the axis transverse to thedirection of light propagation, the wire 52 can be located near thecenter of the waveguide. For example, on each side of device, there canbe a gap 58 between the outer edge of the wire 52 and the outer edge ofthe waveguide 66. This gap 58 can have a width of about 25-30% of thetotal width of the waveguide.

In general, because the dielectric layer 64 below the waveguide 66 andthe empty space or air above the waveguide 66 both have a lowerrefractive index than the waveguide 66, the photons in the waveguide 66are trapped by total internal reflection. However, due to the opticalcoupling between the waveguide 66 and the nanowire 52, the photons canescape into the nanowires 52 and thus be absorbed by the nanowire 62.The light coupling efficiency can be very high in this type of device.

FIG. 9 illustrates a cross-sectional side view of awaveguide-configuration of a superconducting nanowire single photondetector (SNSPD) device 150. The SNSPD device 150 can be similar to thedevices described above with respect to FIGS. 4 and 8 , except asdescribed below.

The SNSPD device 150 includes a substrate 62, such as a siliconsubstrate.

Covering the top surface of the substrate 62 is the dielectric layer 64.The dielectric layer 64 can be silicon oxide (SiO₂), although othermaterials having a refractive index less than that of the waveguide 66are possible. The dielectric layer 64 can have a thickness of at least100 nm, e.g., 200 nm to 2 um.

The waveguide 66 is disposed on the dielectric layer 64. The waveguide66 can be silicon nitride (Si₃N₄), although other materials having arefractive index greater than that of the dielectric layer 64 arepossible. The particular thickness and width of the waveguide can beselected based on the wavelength of light to be captured and detected.The waveguide 66 can have a thickness of 400 to 500 nm, e.g., 450 nm,for 1550 nm light. The width of the waveguide 66, i.e., perpendicular tothe direction of propagation of the light entering the waveguide 66, canbe 1.1 to 1.3 um, e.g., 1.2 um for 1550 nm light.

On the top surface of the waveguide 66, e.g., in direct contact with thewaveguide 66, is a metal nitride seed layer 152. The seed layer 152 andthe superconductive wires 52 are nitrides of different metals. Inparticular, the metal nitride of the seed layer 152 can be aluminumnitride (AlN), as this improves the critical temperature of NbN.However, gallium nitride (GaN) might also be suitable. The seed layer152 can have a thickness of about bout 4 to 50 nm, e.g., about 5 nm orabout 10 nm or about 20 nm thickness. The seed layer 152 can have a(002) c-axis crystal orientation. The seed layer 152 is notsuperconducting at the operating temperature of the device 150.

The superconductive wires 52 are formed on, e.g., in direct contactwith, the seed layer 152. The wires are formed of niobium nitride (NbN),titanium nitride (TiN), or niobium titanium nitride (Nb_(X)Ti_(1-X)N).The wires 52 can have a width of about 25 to 250 nm, e.g., about 60 nm,and a thickness of 4 to 50 nm, e.g., about 5 nm or about 10 nm or about20 nm.

A capping layer 154 can cover the superconductive wires 52. The cappinglayer 154 serves as a protective layer, e.g., to prevent oxidation ofthe metal nitride of the superconductive wires 52 or other types ofcontamination or damage. The capping layer 154 can be dielectric orconductive but is not superconductive at the operating temperature ofthe device 150. The capping layer 154 can be amorphous silicon (a-Si).In some implementations, the capping layer 154 is a nitride of adifferent material from the metal of the metal nitride used for thesuperconductive layer 52. Examples of materials for the capping layer104 include AlN, Al₂O₃, SiO₂, and SiN. The capping layer 104 can bedeposited by a standard chemical vapor deposition or physical vapordeposition process.

Trenches that separate the wires 52 can extend through the capping layer154, the superconductive layer that provides the wires 52, and the seedlayer 152. The trenches need not extend into the waveguide.

FIG. 10 illustrates a cross-sectional side view of another embodiment ofa waveguide-configuration of a superconducting nanowire single photondetector (SNSPD) device 150′. The SNSPD device 150′ can be similar tothe devices described above with respect to FIG. 9 , except as describedbelow.

In the embodiment shown in FIG. 10 , the waveguide 66′ is formed ofaluminum nitride (AlN). Thus a separate seed layer is unnecessary, andthe waveguide 66′ itself acts as the seed layer for the NbN.

The dielectric layer 64 can be silicon oxide (SiO₂), although othermaterials having a refractive index less than that of the aluminumnitride of the waveguide 66′ are possible, e.g., silicon nitride(Si₃N₄). As noted above, the particular thickness and width of thewaveguide can be selected based on the wavelength of light to becaptured and detected. The waveguide 66′ can have a thickness of 400 to500 nm, e.g., 450 nm, for 1550 nm light. The width of the waveguide 66′,i.e., perpendicular to the direction of propagation of the lightentering the waveguide 66′, can be 1.1 to 1.3 um, e.g., 1.2 um for 1550nm light.

The superconductive wires 52 are formed on, e.g., in direct contactwith, the waveguide 66′.

While particular implementations have been described, other and furtherimplementations may be devised without departing from the basic scope ofthis disclosure. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation. It is to be noted, however, that the drawingsillustrate only exemplary embodiments. The scope of the invention isdetermined by the claims that follow.

What is claimed is:
 1. A superconducting nanowire single photon detector(SNSPD) device, comprising: a substrate having a top surface; an opticalwaveguide on the top surface of the substrate to receive lightpropagating substantially parallel to the top surface of the substrate;a seed layer of metal nitride in direct contact with the opticalwaveguide; and a superconductive wire on the seed layer, thesuperconductive wire being a metal nitride different from the metalnitride of the seed layer and optically coupled to the opticalwaveguide.
 2. The device of claim 1, wherein the metal nitride of theseed layer is aluminum nitride.
 3. The device of claim 1, comprising adielectric layer of a first material having a first index of refractionbetween the substrate and the optical waveguide, wherein the opticalwaveguide is formed of a second material having a second index ofrefraction greater than the first index of refraction.
 4. The device ofclaim 3, wherein the first material is silicon oxide (SiO₂).
 5. Thedevice of claim 3, wherein the second material is silicon nitride(Si₃N₄).
 6. The device of claim 2, wherein the metal nitride of thesuperconductive wire is niobium nitride, titanium nitride, or niobiumtitanium nitride.
 7. The device of claim 6, wherein the metal nitride ofthe superconductive wire comprises δ-phase NbN.
 8. The device of claim1, further comprising a capping layer on the superconductive wire. 9.The device of claim 8, wherein the superconductive wire includes aplurality of wire portions, and trenches that separate the plurality ofwires portions extend through the capping layer.
 10. The device of claim1, wherein the seed layer has a thickness of 4 to 50 nm.
 11. The deviceof claim 1, wherein the superconductive wire has a thickness of 4 to 50nm.
 12. The device of claim 1, wherein the superconductive wire includesa plurality of wire portions, and trenches that separate the pluralityof wires portions do not extend into the optical waveguide.
 13. Thedevice of claim 12, wherein the trenches extend into the seed layer.